Refractory fibers are inorganic materials formed from melts composed of inorganic oxides, predominantly alumina and/or silica. In the conventional method of forming refractory fibers, a mixture of oxides is melted and attenuated into elongated fibers. The fibers are collected on a moving belt in the form of a low density, fluffy, continuous, blanket-like body. Thereafter the collected body of fibers can be used as a bulk fiber source, reformed into a denser mat or blanket, or broken up to form short length fibers for use in molding processed. The high temperature resistance of the refractory fiber products makes them extremely valuable for use as thermal insulation in high temperature environments such as furnaces, kilns, ovens and numerous other high temperature applications where bulk or formed insulation is required. When the original oxide mixture is formed almost entirely of generally equal amounts of silica and alumina, the resulting fibers normally are resistant to temperatures up to about 2400.degree. F. (1315.degree. C.). Addition of up to 10 to 20 percent by weight of other oxides in different combinations to the basic alumina/silica mixture can produce fibers of differing degrees of thermal resistance. Variation of the ratios of silica and alumina will also affect the thermal resistance, with those fibers having greater proportions of alumina being more thermally resistant than the fibers having higher proportions of silica. Commercial fibers are available for service in the range of from 1200.degree. F. (650.degree. C.) up to approximately 3000.degree. F. (1650.degree. C.). Typical of the commercial refractory fibers in the market place are those sold by Johns-Manville Corporation under the trademarks CERAWOOL, CERAFIBER and CERACHROME.
When the fibrous blanket is formed, the fibers lay down in a straight, non-interlocking configuration. Thus, the raw blanket is relatively weak and difficult to handle. Various mechanical means are used to cause strengthening by interlocking of fibers. One such means is the use of unthreaded barbed needles which force a number of fibers vertically through the blanket. However, the increase in strength which can be obtained by such mechanical means is rather limited.
Further, the use of mechanical needling does not yield blankets of good dimensional precision. Fibrous insulating blankets are normally sold on the basis of their thickness and density, which together indicate the thermal resistance properties of the blankets. Mechanical needling usually produces blankets which may have dimensional variations of 25% or more from the desired nominal thickness. Since the quantity of fibers incorporated in the blanket per unit length is controlled on the basis of nominal thickness, the variation in thickness also causes a density variation and thus a variation from the desired thermal properties. It is known that the initial attenuated fiber is formed with differential residual stress present throughout the fiber. It is also known that an elevated temperature stress relief will cause differential relief of these stresses causing the stress relieved fiber to curl. Such a heat treatment results in effective and thorough interlocking of the curled fibers throughout the blanket cross section resulting in quite effective strengthening of the blanket. The heat treated blanket is thus quite capable of being readily handled and installed in normal thermal insulation applications.
In the past it has been the practice to stress relieve the fibers by annealing the refractory fiber blankets in an oven. Typically, this is accomplished by slowly moving the refractory fiber blankets through a long heated tunnel where each is continuously exposed to a high temperature environment. Conventionally, such annealing requires a minimum of at least about 5 minutes in the oven and more commonly requires a half hour or more.
There are significant disadvantages and problems with conventional heat soak annealing. The long time periods required seriously reduce the amount of annealed fiber production which can be obtained unless extremely long heat soak tunnels are used to compensate for the long residence time. Use of long tunnels, however, raises the problem of high cost and substantial space requirements. Further, in the heat soak tunnel environment there are usually no restraints on the blankets so that the annealing process can result in dimensional distortion of the blankets within the tunnel. Since such finished annealed blankets therefore would have irregular dimensions, their subsequent installation in company with other annealed blankets in an oven or similar device is seriously complicated, for the installer must take into account the unique dimensional irregularities of such individual blankets.
In addition, the heat soak tunnels require the use of large amounts of energy. It would therefore be of significant value to have a process in which refractory fiber bodies such as blankets could be annealed rapidly while yet maintaining and assuring dimensional integrity and reducing energy consumption.